(1) Field of the Invention
The present invention relates to an image forming apparatus and a tone correction method, in particular to a technology for performing a tone correction with high accuracy.
(2) Description of the Related Art
In order to realize high image quality, a tandem-type color image forming apparatus performs a process such as a registration correction process or tone correction process as an image stabilization process. It is desirable that time required to perform the image stabilization process be as short as possible, since a delay in an image formation due to the image stabilization process affects users' convenience.
In response to the above need, for example, a technique has been developed to perform a color shift correction control process and a halftone density control process simultaneously in a parallel manner by forming a color shift detection pattern and a halftone density detection pattern on a transport belt (Japanese Laid-Open Patent Application Publication No. 2002-014505). Also, another technique has been developed to form, during a tone correction, on an intermediate transfer belt, a tone patch whose tone varies continuously (Japanese Laid-Open Patent Application Publication No. 2004-077873).
However, when forming a color shift detection pattern on the transport belt or forming a density patch on the intermediate transfer belt, speed of the transport belt or the intermediate transfer belt may vary, causing a shift in a forming position of the color shift detection pattern or density patch. An occurrence of such a positional shift hinders detection of an expected density and thereby an accurate tone correction, preventing the realization of the high image quality as a result.